1. Field of the Invention
The invention relates to an intake air flow rate control device for an internal combustion engine, which has a function of calculating a target throttle opening degree on the basis of a required intake air flow rate of the internal combustion engine.
2. Description of the Related Art
In control over the intake air flow rate (air flow rate taken into a cylinder) of an internal combustion engine, there is a technique for compensating for a delay in response of an intake system (for example, a delay in response of a throttle valve or a delay in response due to the volume of an intake passage). For example, Japanese Patent Application Publication No. 2006-70701 (JP-A-2006-70701) describes that a target intake air flow rate of the internal combustion engine is converted to a target intake air flow rate that may be achieved by a normative model and then the inverse model of a model that takes a delay in response of the intake system into consideration is used to calculate a target throttle opening degree so that the output of the normative model coincides with an actual intake air flow rate (or an estimated intake air flow rate).
In addition, Japanese Patent No. 3873608 describes that the ratio of opening areas before and after movement of a throttle valve is calculated on the basis of a target intake pipe pressure calculated from a target intake air flow rate of an internal combustion engine and then a target throttle opening degree after movement of the throttle valve is calculated on the basis of the ratio of opening areas before and after movement of the throttle valve and an opening area before movement of the throttle valve.
Incidentally, when the inverse model of an intake system model that takes a delay in response of the intake system into consideration is used to calculate a target throttle opening degree from a required intake air flow rate, the target throttle opening degree tends to vary with oscillations. As the target throttle opening degree oscillates, the actual throttle opening degree oscillates and, therefore, the variation amount of the actual throttle opening degree increases. This may increase the amount of wear of a sliding portion of a throttle device formed of, for example, a throttle valve and a motor that drives the throttle valve to decrease durability or may cause the motor that drives the throttle valve to overheat.
Then, Japanese Patent Application Publication No. 2006-200466 (JP-A-2006-200466) describes that the inverse model of an intake system model that takes a delay in response of the intake system into consideration is used to calculate a target throttle opening degree from a required intake air flow rate during transitional operation of an internal combustion engine; whereas a target throttle opening degree is calculated on the basis of the operating state of the internal combustion engine without using the inverse model of the intake system model during steady operation of the internal combustion engine.
In the technique described in JP-A-2006-200466, a target throttle opening degree is calculated on the basis of the operating state of the internal combustion engine without using the inverse model of the intake system model during steady operation of the internal combustion engine, so oscillation of the target throttle opening degree may be suppressed. However, during transitional operation of the internal combustion engine, a target throttle opening degree is calculated from a required intake air flow rate using the inverse model of the intake system model in order to compensate for a delay in response of the intake system, so oscillation of the target throttle opening degree cannot be suppressed, and it is difficult to eliminate inconvenience due to oscillation of the target throttle opening degree.
The invention provides an intake air flow rate control device for an internal combustion engine, which is able to set a target throttle opening degree that is compensated for a delay in response of an intake system while suppressing oscillation of the target throttle opening degree even during transitional operation of the internal combustion engine.
A first aspect of the invention relates to an intake air flow rate control device for an internal combustion engine, which has a function of calculating a target throttle opening degree on the basis of a required intake air flow rate of the internal combustion engine. The intake air flow rate control device includes: first required throttle opening degree calculation means that calculates a first required throttle opening degree in consideration of a delay in response of an intake system on the basis of the required intake air flow rate; second required throttle opening degree calculation means that calculates a second required throttle opening degree in consideration of a delay in response of the intake system on the basis of the required intake air flow rate in a response characteristic different from that of the first required throttle opening degree calculation means; and target throttle opening degree calculation means that calculates the target throttle opening degree on the basis of the first required throttle opening degree and the second required throttle opening degree, wherein the target throttle opening degree calculation means sets the target throttle opening degree so that the target throttle opening degree falls within the range from the first required throttle opening degree to the second required throttle opening degree and a variation amount of the target throttle opening degree is smaller than a variation amount of any one of the first and second required throttle opening degrees.
With the above configuration, the first required throttle opening degree and the second required throttle opening degree are calculated from the required intake air flow rate in consideration of a delay in response of the intake system, one of the first and second required throttle opening degrees is set as a low-response side allowable limit value of the required throttle opening degree, and the other one is set as a high-response side allowable limit value of the required throttle opening degree. By so doing, it is possible to set an allowable range of the required throttle opening degree that is compensated for a delay in response of the intake system. Then, the target throttle opening degree is set so as to fall within the range from the first required throttle opening degree to the second required throttle opening degree (that is, allowable range of the required throttle opening degree that is compensated for a delay in response of the intake system). By so doing, the target throttle opening degree that is compensated for a delay in response of the intake system may be set within the allowable range. Furthermore, the target throttle opening degree is set so that a variation amount of the target throttle opening degree is smaller than a variation amount of any one of the first and second required throttle opening degrees. By so doing, oscillation of the target throttle opening degree may be suppressed. In this Way, the target throttle opening degree that is compensated for a delay in response of the intake system may be set while suppressing oscillation of the target throttle opening degree even during transitional operation of the internal combustion engine.
The first required throttle opening degree calculation means and the second required throttle opening degree calculation means each may have a function of converting the required intake air flow rate to a required intake air flow rate, of which a response may be achieved by a predetermined normative model, (hereinafter, referred to as “post-model required intake air flow rate”), and calculating the required throttle opening degree so as to achieve the post-model required intake air flow rate using the inverse model of an intake system model that takes a delay in response of the intake system into consideration, a response characteristic of the normative model in the first required throttle opening degree calculation means may be different from a response characteristic of the normative model in the second required throttle opening degree calculation means. By so doing, the response characteristic of the required throttle opening degree may be changed by a simple method that the response characteristic (for example, time constant) of the normative model is varied between the first required throttle opening degree calculation means and the second required throttle opening degree calculation means.
In addition, when the target throttle opening degree is set on the basis of the first required throttle opening degree and the second required throttle opening degree, the intake air flow rate control device may further include throttle opening degree estimating means that calculates an estimated throttle opening degree, which is an estimated actual throttle opening degree, on the basis of a last value of the target throttle opening degree, wherein the target throttle opening degree calculation means may set the estimated throttle opening degree as the target throttle opening degree when the estimated throttle opening degree falls within the range from the first required throttle opening degree to the second required throttle opening degree, and may set one of the first required throttle opening degree and the second required throttle opening degree, which is closer to the estimated throttle opening degree, as the target throttle opening degree when the estimated throttle opening degree falls outside the range from the first required throttle opening degree to the second required throttle opening degree. By so doing, it is possible to reduce a variation amount of the target throttle opening degree while maintaining the target throttle opening degree within the range from the first required throttle opening degree to the second required throttle opening degree. In this case, a deviation between the estimated throttle opening degree (estimated actual throttle opening degree) and the target throttle opening degree may be reduced, so the variation amount of the actual throttle opening degree may be effectively reduced. Thus, it is possible to effectively improve durability by reducing the amount of wear of a sliding portion of a throttle device.
Alternatively, the target throttle opening degree calculation means may set a last value of the target throttle opening degree as a current value of the target throttle opening degree when the last value of the target throttle opening degree falls within the range from the first required throttle opening degree to the second required throttle opening degree, and may set one of the first required throttle opening degree and the second required throttle opening degree, which is closer to a last value of the target throttle opening degree, as a current value of the target throttle opening degree when the last value of the target throttle opening degree falls outside the range from the first required throttle opening degree to the second required throttle opening degree. By so doing as well, it is possible to reduce a variation amount of the target throttle opening degree while maintaining the target throttle opening degree within the range from the first required throttle opening degree to the second required throttle opening degree. In this case, a deviation between the last value and current value of the target throttle opening degree may be reduced, so the variation amount of the target throttle opening degree may be effectively reduced. Thus, fluctuations in electric current flowing through the motor that drives the throttle valve are suppressed to thereby make it possible to effectively prevent overheating of the motor.